Revert "Rename to PanelToplevel"

This reverts commit d2c3901f6ffb0aaf5e291ab2b2567e84cab6a1dd.

Simply put, we do not want to support gnome panel themes anymore, as
the panel will take on a transparent appearance, with certain styled
elements. Supporting legacy theming not truly compatible with our own
panel will create untold conflicts, and is not something we wish to
support here.